Method of and apparatus for depositing substances by thermal evaporation in vacuum chambers



Apnl 20, 1948. P. ALEXANDER METHODS OF AND APPARATUS FOR DEPOSITING SUBSTANCES BY THERMAL EVAPORATION IN A VACUUM CHAMBER Filed Sept. 15, 1944 F/GZ FIGS Patented Apr. 20, 1948 METHOD OF AND APPARATUS FOR DE- POSH'ING SUBSTANCES m EVAPORATION IN VACUUM Paul Alexander, Berkhamated, England lieation September 1 1944, Serial No. 554.81: In Great Britah r-Augnat 4, 1m

12 Claims. (01. 117-105) 1 This invention relates to methods of and apparatus for depositing a coating oi metal or of silica, or other non-metallic substances, progressively on a support by thermal evaporation in a vacuum chamber.

The process is applicable to substances 0! the type which have a vapour pressure oi at least 1 mm. Hg at a temperature which can be realised in a container in which the substance is vapourised and whichcan exist in the form of vapo at the temperature. a

In the thermal deposition oi materials in a vacuum the quality oi. the deposit is not good it the vapour reaches the support in the form of molecular aggregations. Metals evaporate generally in atomic form, though in certain cases they may evaporate as single molecules or partly as atoms and partly as molecules. Moreover silica is evaporated in molecules. For the sake of clarity the word atom" will be used in this specification to include molecules of a material being deposited and the term molecules" is used only in relation to the residualgas in the vacuum chamber.

The aggregations which produce deposits oi bad quality consist of large numbers of atoms. Atoms of metals or silica, on colliding, tend to coalesce into aggregations. This tendency is less the greater the kinetic energy of the atoms. When the vapourised material issues into a space containing gas (usually air) molecules, collisions occur between the atoms and the gas molecules,

and the atoms thereby lose kinetic energy. and

are then more liable to coalesceinto aggregations on colliding with other atoms.

For this reason, in order to obtain a deposit of good quality, the deposition has been carried out in a chamber at a high vacuum of 10" to 10* mm. of mercury, when evaporating metal or silica from a bath. In such a vacuum the gas molecules are greatly reduced in number, and the chance of a collision between an atom of metal or silica and a gas molecule is correspondingly reduced.

With a view to obviating the need ior a high vacuum which, because oi the long process 01 evacuation required. greatly increases the time required to efiect deposition and its cost, a low vapour density of the metal or silica has been maintained in the chamber, so that, even if the kinetic energy of the atoms is reduced tolow values, the chance of atoms colliding is small. This altematlve method is fully eiiectlve tor the purpose in view, but the rate of deposition is necessarily low by reason of the low vapour density maintained during the process.

A main obiect oi the present invention is to produce a deposit or good quality at a high rate under low vacuum conditions in an economical manner.

Another main object is to create a Jet or beam 0! the vapour in which the partial pressure 01 the residual gas is substantially less than that which obtains in. the vacuum chamber.

Another object is to provide improved means of heating the material to be deposited, and its vapour.

A method of progressively depositing a coating of a substance 0! the type described, on a moving support, by thermal evaporation in a vacuum chamber, in accordance with the present invention consists in producing a low vacuum in the chamber, vapourising the substance to be deposited in a container, confining the vapour so generated to obtain a pressure in the vapour which is high relative to that in the vacuum chamber. releasing the high pressure vapour from the container into the chamber as a beam whereby the potential energy in the vapour is substantially converted into kinetic energy in the form of directional velocity of the atoms in the beam, which velocity is high relatively to the mean random velocity oi the atoms, whereby residual gas molecules in the chamber and adiacent to the beam are substantially prevented from entering the body of the beam, disposing the support in the path of the high velocity beam and moving the support across the beam.

The support may be screened from impact of the atoms in the outer fringe of the beam, but preferably the method of depositing according to the invention is further characterised by shielding the path of the beam to form a depositing enclosure into which the beam is pro- Jected at an angle to the normal to the support. the depositing enclosure giving passage from the enclosure to the vacuum chamber for the portion of the atoms in the beam deflected irom the support and restricting ingress of the residual gas molecules irom the vacuum chamber into the enclosure at points other than the said passage.

The invention comprises also apparatus for depositing on a support a coating of a substance of the type described by thermal evaporation in a vacuum chamber, which comprises means for vapourising the substance including a container and heating means therefor, means for producin a beam of atoms oi the substance directed II with high velocity against the support, consistcontainer and directed towards the support, each said nozzle by gradually increasing in cross-sectional area from the container to its mouth, whereby the potential energy of the stream of atoms entering the nozzle is substantially converted into kinetic energy of the beam issuing from the nozzle, and the container being closed except for the nozzle aperture, and means for progressively moving the support across the beam.

Preferably as hereafter pointed out a flat beam is produced by utilising at least one slit nozzle, the nozzle being directed towards the support at an angle to the normal of the support, and the apparatus comprises an enclosure within the vacuum chamber into which the beam issues and in which the depositing is effected, the enclosure having an aperture communicating with the' vacuum chamber placed in the path of atoms of the beam deflected from the support, and being otherwise closed except for leakage gaps.

In order that the invention may be more clearly understood preferred embodiments thereof will now be'desoribed with reference to the accompanying drawings.

In the drawings:

Figure 1 is a vertical section through the apparatus;

Figure 2 is a horizontal section of the heating device along the line A-A of Figure 1;

Figure 3 is a vertical section lengthways of one form of nozzle;

Figure 4 is a plan view thereof;

' Figure 5 is a part vertical section through an alternative form of apparatus;

Figure 6 is a section along the line BB of Figure 5;

Figures 7 and 8 are cross sections of alternative forms of nozzles.

Referring to Figures 1' and 2, in one method of depositing a coating according to the invention.

the vacuum chamber i contains the support 2 on which the coating is to be made and a container 3 in which the substance 4 to be evaporated is contained. The substance 4 may be heated to above its vaporisation point in any convenient known way. In a preferred way, according to the invention, a tube 5, whichmay beof the same material as the containenpasses through the container, making a vapour-tight joint in passing through the walls at 6. An electric heater 1 passes through the tube without touching it and is supplied'with current from terminals 8. The tube is heated by radiation from the heater and this, in turn, heats the substance 4 to be evaporated and the container 3 by radiation.

The container ends upwardly in a neck 9 which is closed by a block ID in which is formed a nozzle H which widens upwardly from an extremely narrow slit I! through which the vapour issues from the container. Heat is supplied by the tube 5 to the substance 4 at a rate suiliclent to produce a vapour pressure in the container which is high relative to the pressure in the vacuum chamber.

By reason of the fall in pressure along the nozzle, the potential energy of the vapour in the container is converted into kinetic energy of the stream of atoms, and consequently the vapour issues from the nozzle in the form of a diverging jet or beam 13 of high velocity atoms. The nozzle Ii is directed to the support 2, so

4 that the beam l3 impinges directly on the support.

Residual gas molecules in the vacuum chamber which, in their random motions strike the beam l3 are swept along by the high velocity atoms in the outer fringe of the beam, and are unable to enter the body of the beam. Therefore the atoms in the body of the beam retain their kinetic energy and do not coalesce into molecular aggregations, and the deposit made by them on the support is of good quality.

The support shown by way of example is a strip of paper 2 which passes from the supply roll I! to the take-up roll is over idle rollers IS, the take-up roll being driven by clockwork indicated at Ilia. c,

If the pressure in the vacuum chamber is high relatively to the pressure commonly used in thermal evaporation, for instance, 0.01 mm. of mercury, the residual gas molecules will strike the beam in such quantity that the atoms in the outer fringe of the beam will lose their kinetic energy and will coalesce with one another, and therefore the deposit formed by the outer fringe of the beam will not be of good quality. To avoid the formation on the support of a deposit of inferior quality, the fringes of the beam are prevented from striking the support by interposition of a screen I].

The penetration into the beam of gas molecules in sufllcient number to impair the quality of the deposit may be made as small as desired by increasing the pressure fall along the nozzle, so as to increase the velocity of theatoms, or by diminishing the divergence of the beam so as to increase its density, or by lowering the pressure in the vacuum chamber so as to diminish the number of molecules striking the beam.

The beam is given a form such that it strikes the support 2 along a strip extending across the support, as determined by the screen H. The support, then, is progressively coated with deposit as it passes from roll ll to roll l5. 1

The nozzle block i0 must be maintained at a temperature sufficient to prevent condensation of the vapour in the nozzle. Preferably it is heated by means of heating elements I. To the same end, the tube 8 is preferably placed in the passage of the vapour from the substance 4 to the nozzle. Since heat is transferred from the tube to the substance 4, the temperature of the tube is higher than that of the substance and therefore also higher than that of the vapour evolved from the substance. The vapour, therefore, in passing over the tube on its way to the nozzle, is superheated by the tube, and is then less liable to be condensed in the nozzle.

In order to diminish loss of heat from the container and nozzle block to the walls of the vacuum chamber, the container and nozzle block are preferably jacketed by a plurality of heat refleeting screens i9. These are metal sheets with surfaces of good heat reflecting power, for instance, polished copper sheets. Heat transmission from one screen to another and to the vacuum chamber is reduced to a minimum by providing supports 20 of very small cross-section, so that the heat absorbed by each screen is only that portion of the heat radiated from the container and nozzle block, or from the next inner screen, which is not reflected.

The nozzle may, by way of example, have walls with a total divergence angle of 10 degrees, and a width of slit I 2 of 0.03 inch. Its length may be suflicient to produce a beam extending across the support. Alternatively, the nozzle block I! may be formed with a plurality of nozzles II, as shown in Figures 3 and 4. The beams irom the individual nozzles then coalesce into a single beam at the outlet of the nozzle block.

Suitable pressures are from 5 to mm. in the container and 0.1 mm. in the vacuum chamber. The pressure in the container may be as low as 1 mm. but, with the resulting beam 0! lower velocity and density, a lower pressure in the vacuum chamber is advisable. A suitable distance between the nozzle and the support may be about 2 inches.

Still higher pressures may be used in the vacuum chamber with the apparatus shown in Figures 5 and 6, in which the heating device is omitted, as it may be similar to that shown in Figure 1. A deposition enclosure is formed within the vacuum chamber by side walls 22 and 23 and end walls 24; this enclosure communicates with the vacuum chamber through an outlet 25, but is otherwise closed. In the example shown, the support is a roll of paper 2 which is fed from the supply roll it to the take-up roll l5 over a former 25, in the form of a roller. The closure of the deposition enclosure is completed by the end walls 24 being in contact with the end of the former 26, and by the wall 23 having a portion 21 which is very close to the support.

The former is so placed relatively to the beam l3 and the outlet 25 that the beam impinges on the support at an angle to the normal 28 at the central line of impact, so that a portion of it is deflected in the direction of the outlet 25. This deflected portion of the beam carries with it residual gas molecules from the deposition enclosure into the vacuum chamber, and thus evacuates the deposition enclosure to a pressure lower than that in the vacuum chamber. The stream of atoms and molecules passing through the outlet 25 is deflected from the support by the battle 29.

It is found that a partial pressure of the residual gas of 10- mm. can be maintained in the beam, with a pressure in the vacuum chamber of 0.1 mm., and with about 10% of the beam deflected through the outlet 25, provided that the leakage gaps, as at 21, do not greatly exceed the mean free path of the gas molecules, namely about 0.7 mm. in the case of a pressure of 0.1 mm.

By way of example, to deposit zinc on a lon strip of paper, the zinc is heated in a steel container to a temperature of about 575 C., thereby producing a vapour pressure in the container of about 5 mm.; the nozzle is maintained at a temperature above 500; the pressure in the vacuum chamber is maintained at 0.1 mm.; the paper strip is passed over the former at the rate of 2000-3000 yards per hour. A bright deposit of good quality is then obtained about 0.2 micron thick.

Instead of using a nozzle with walls diverging at about 10 degrees total angle, a beam of greater density for the same pressure in the container may be obtained by diminishing the total angle, or by using a twin nozzle as shown in Figure 7, each nozzle having walls diverging at about 5 degrees total angle. A beam of still greater density may be obtained by a twin nozzle as shown in Figure 8, in which the two outer walls of the nozzle are parallel and directed towards the support, while the inner walls are inclined to them at an angle of, say, 5 degrees.

For metals such as cadmium, zinc, magnesium and lead, whichhave a higher vapour pressure nozzle in such a way that gas molecules strikin the beam are swept aside by the momentum of the atoms and in consequence of the density of the atoms in the beam, and the coating is formed under conditions such that locally a. low partial pressure of gas is created in the body of the beam though a moderately low pressure only obtains in the vacuum chamber. Accordingly a bright film can be obtained at a rapid rate without encountering a long process of producing a high vacuum in the vacuum chamber.

I claim:

1. Apparatus for depositing on a support a coating of a substance 01' the type described by thermal evaporation in a vacuum chamber, comprising means for vapourising the substance in-- cluding a container and heating means therefor, a nozzle structure mounted on the container for producing a beam of atoms of the substance directed with high velocity against the support. including at least one nozzle emanating from the container and directed towards the support, each said nozzle gradually increasing in crosssectional area from the container to itscmouth, whereby the potential energy of the stream of atoms entering the nozzle is substantially converted into kinetic energy of the beam issuing from the nozzle, and the container being closed except for the nozzle aperture, and means for progressively moving the support across the beam.

2. Apparatus for depositing on a support a coating of a substance of the type described, by thermal evaporation in a vacuum chamber, comprising means for vaporising the substance including a container and heating means therefor, means for producing a that beam of atoms of the substance directed with high velocity against the support, including a nozzle structure having at least one slit nozzle emanating from the container and directed towards the support. each said nozzle gradually increasing in crosssectional area from the container to its mouth, whereby the potential energy of the stream of atoms entering the nozzle is substantially converted into kinetic energy of the beam issuing from the nozzle, and means for moving the support in a direction at right angles to the long dimension of the beam, the container being closed except for the nozzle aperture.

3. Apparatus for depositing on a support a coating of a substance of the type described, by thermal evaporation in a vacuum chamber, comprising means for vaporising the substance, including a container and heating means therefor, means for producing a beam of atoms of the substance directed with high velocity against the support including a nozzle structure having at least one nozzle emanating from the container and directed towards the support at an angle to the normal to the support, each said nozzle gradually increasing in cross-sectional area from the container to its mouth, whereby the potential energy of the stream of atoms entering the nozzle is substantially converted into kinetic energy of the beam issuing from the nozzle, the container being closed except for the nozzle aperture, an enclosure within the vacuum chamber into which the beam issues and in which the deposition is eflected, the enclosure having an aperture communicating with the vacuum chamber placed in the path of atoms oi the beam deflected from the support, and being otherwise closed except for leakage gaps, and means i'or/ progressively moving the support across the the width oi the support to be coated, a neck' to the container extending tor the length thereof, a metal tube making a vapour tight Joint with the end walls of the neck, heating means in the tube to vaporise said material in the container and heat the vapour passing through the neck, .a nozzle structure extending lengthwise or the neck to form a diverging flat beam substantially adapted to convert the potential energy ot the vapour in the container into kinetic energy 01' the beam, and means for progressively moving the support across the beam, an enclosure about said beam defined by connected shields extending from the nozzle structure in the direction of the beam to the path taken by the support in passing across said beam, means ior disposing the impact area of the beam at an inclination to the beam, said connected shields including a shield which emanates from that side so of the nozzle structure facing the obtuse angle acterised by means for vaporising the material to be deposited including a container and heating means therefor, a nozzle structure mounted on the container, the structure having at least one nozzle each nozzle gradually increasing in cross-sectional area from the container to its mouth, whereby the potential energy 01 the stream oi atoms entering the nozzle is substantially converted into kinetic energy of the beam formed by the nozzle arrangement, the container being closed except for the nozzle structure means for progressively moving the support across the beam, a depositing enclosure defined by a shield arranged about the path of the beam extending from the container to the path of the support and the nozzle structure being such that a diverging fiat beam may be directed against the support, the shield being apertured on one side of the beam to provide a tree outlet near the support for gas molecules which may arrive in the enclosure through the leak inlet which occurs on the otherside oi the beam path between the shield and the moving support.

6. Apparatus for depositing on a support a coating of a substance oi the type described, by thermal evaporation in a vacuum chamber, characterised by means for vaporising the material to be deposited including a container and heating means therefor, a nozzle structure mounted on the container whereby a diverging fiat beam may be directed against the support, an enclosure formed about the path of said beam defined by a shield arranged about said path and extending from the nozzle structure to the path 01' the support across said beam, the shield being apertured on one sideof the beam to provide an outlet near the support for gas molecules which may arrive in the enclosure through aleak inlet on the, opposite side of the beam path between the shield and the moving support, and means for disposing the impact area oi the support at an inclination to the beam so that all gas molecules in the vicinity of said area are progressively driven out by the beam directly towards said outlet and a local low partial air pressure created at the area of impact.

.7. Apparatus for depositing on a support a coating or a substance of the type described, by thermal evaporation in a vacuum chamber, characterised by means for vaporising the material to be deposited including an elongated container having a length substantialy equal to made between the beam and said impact area and which is apertured to readily pass gas molecules driven away from the contact area by the said beam, whereby a local low partial air pressure is created over the area of impact.

8. Apparatus as in claim 1 characterised by a metal screen substantially surrounding the container in which the material to be deposited is vaporized except for the nozzle structure and a second metal screen surrounding the first screen but spaced therefrom, the inner surface oi each screen being oi good heat reflecting power.

9. Apparatus as in claim 5 characterised by a metal screen substantially surrounding the container in which the material'to be deposited is vaporised except for the nozzle structure and a second metal screen surrounding the first screen but spaced therefrom, the inner surface of each screen being oi good heat reflecting power.

10. Method 01' progressively depositing a coating of a substance of the type described, on a moving support, by thermal evaporation in a vacuum chamber, consisting in producing a low vacuum in the chamber, vapourising the substance to be deposited in a container, confining the vapour so generated to obtain a pressure in the vapour which is high relative to that in the vacuum .chamber, releasing the high pressure vapour from the container into the chamber as a beam whereby the potential energy in the vapour is substantially converted into kinetic energy in the form of directional velocity 0! the atoms in the beam, which-velocity is high relatively to the mean random velocity oi the atoms, whereby residual gas molecules in the chamber and adjacent to the beam are substantially prevented irom entering the body 01 the beam, disposing the support in the path of the high velocity beam and moving the support across the beam.

11. Method oi progressively depositing a coating of a substance of the type described, on

. a moving support, by thermal evaporation in a vacuum chamber, consisting in producing a low vacuum in the chamber, vapourising the substance to be deposited in a container, confining thevapour so generated to obtain a pressure in the vapour which is high relative to that in the vacuum chamber, releasing the high pressure vapour from the container into the chamber as a beam whereby the potential energy in the vapour is substantially converted into kinetic energy in the form of directional velocity of the atoms in the beam which velocity is high relatively to the mean random velocity or the atoms,

whereby residual gas molecules in the chamber and adjacent to the beam are substantially prevented from entering the body of the beam, disposing the support in the path of the high velocity beam, moving the support across the beam, and screening the support from impact of the atoms in the outer fringe of the beam, whereby substantially only atoms which have not collided with residual gas molecules impinge on the support.

12. Method of progressively depositing a coating of a substance of the type described, on a moving support, by thermal evaporation in a vacuum chamber, consisting in producing a low vacuum in the chamber, vapourising the substance to be deposited in a container, confining the vapour so generated to obtain a pressure in the vapour of at least 1 mm. of mercury, releasing the high pressure vapour from the container into the chamber as a beam under a pressure drop of at least 0.9 mm. of mercury whereby the poten- 2o tial energy in the vapour is substantially converted into kinetic energy in the form of directional velocity of the atoms in the beam, which velocity is high relatively to the mean random 25 10 velocity of the atoms, whereby residual gas molecules in the chamber and adjacent to the beam are substantially prevented from entering the body of the beam, disposing the support in the path of the high velocity beam and moving the support across the beam.

PAUL ALEXANDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,256,599 Schoop Feb. 19, 1918 2,074,281 Sommer Mar. 16, 1937 2,100,045 Alexander Nov. 23, 1937 2,242,101 Atlee May 13, 1941 2,273,941 Dorn Feb. 24, 1942 FOREIGN PATENTS Number Country Date 485,965 Great Britain May 27, 1938 551,757 Great Britain Mar. 9, 1943 

